The present invention is directed to a method for removing a photoresist. More specifically, the present invention is directed to a method of removing a photoresist that has been stressed by plasma containing fluorine and that lies over metallically conductive layers.
Photoresists are utilized for generating structures, particularly interconnect structures in microelectronics. To this end, photoresists are light-sensitive compounds which upon irradiation by a suitable light, or other radiation source, change their chemical structure. These photoresists therefore exhibit a modified solubility behavior in comparison to nonirradiated lacquer areas. By utilizing a suitable developing process, the structures of the lacquer layer that are latently present after irradiation through a mask, can be three-dimensionally fashioned in that the more easily soluble portions of the photoresist layer that, dependent upon the photoresist, can be the exposed or unexposed parts, are dissolved away.
Photoresist structures generated in this manner can function as a mask during further structuring steps of the layers lying therebelow. For example, an etching process can be performed wherein the portions of the layer lying therebelow, that are not covered by the photoresist, are selectively etched.
After this further structuring step, the photoresist layer must again be removed. However, because during the etching process the photoresists are subjected to structural and chemical modifications, the removal of the photoresist layer can present some problems. In this regard, the thermic stressing of the photoresist, the composition of the etching plasma utilized, as well as the ultraviolet hardening of the photoresist, significantly effect the ability to remove the photoresist.
If an etching plasma is utilized that contains fluorine (a process gas, for example, of CF.sub.4 or SF.sub.6) fluorine radicals and other reactive etching gas fragments (for example, CF.sub.3, SF.sub.4, SF.sub.2, COF.sub.2, SOF.sub.2, etc.) will cause a partial or perfloridation of the resist surface (in this regard, one also speaks of a teflonization). The metal fluorides formed during the etching process, acting as Friedel-Craft catalysts, can contribute to an additional cross linking of the photoresist structure.
Furthermore, it is possible that additional compounds will form in the photoresist in view of the metal fluorides acting as Lewis acids. This results in the organic photoresist now having an inorganic component and creating a photoresist that cannot be removed in oxygen plasma without leaving residue. In extreme cases, the modifications to the photoresists can effect the complete resistance of the resist structure, after etching, to solvents (resist strippers) such as Cellosolve or acetone, that are typically utilized.
A photoresist structure treated with fluorine plasma (for example, a situation wherein SF.sub.6 is utilized as the etching gas) will exhibit these properties only at the surface that is exposed to the plasma attack. Accordingly, the inside structure of the photoresist remains easily soluble.
In removing the resists, a large portion of the resist is removed by driving in a solvent or by ripping the outside layer open. The selective stripping of the "inner" resist layers, however, leads to the creation of resist skins that initially only adhere at the edges of the structure. But, after the soluble inside layers of the resist are extracted, the remaining resist skins sink down to the base and, particularly after an evaporation of this surface, the resist skins begin to adhere surface-wide to the base. After this has occurred, the resist skins, or residues, can no longer be removed in a wet-chemical manner utilizing am organic solvent, due to the great adhesion the resist skin exhibits and because of the insolubility of the resist skin.
The above problems are particularly prevelant in large-area resist structures (for example, terminal lugs of interconnect structures) or with especially fine structures having an unfavorable ratio of adhesion points to the remaining resist surface. Whereas, in large-area resist structures, the partially stripped resist skin collapses in an uncontrollable fashion over the etched structure and thus contaminates the wafer surface, especially thick skins have been observed in the case of fine structures.
For these reasons, the conventional removal, after a plasma etching step, of the photoresist is of special importance. After acetone removal (even after a preceding pre-treatment step with oxygen plasma), the etched metal structure is still covered by a solid resist coating at many locations. The resist can partially break open at the upper edge of especially fine meandered resist structures. This results in the creation of resist halves that fold into the interspaces not covered by the resist and, for example, an undesired mask during a subsequent ion beam etching.
It is believed that such resist residues that have inorganic components, can only be converted into a purely organic compound by utilizing an additional acid treatment. This purely organic compound or phase then being oxidizable in an oxygen plasma and, thus, being removable.